This research will investigate the structure and acquisition of American Sign Language (ASL), a natural language of the deaf of North America. ASL may be acquired as a primary or second language, either early or late in life, and either from native signers or from signers who themselves acquired the language late in life. Its study therefore provides an unusual opportunity to investigate the consequences of early vs. late experience, and of wide variations in input environment, on the process by which a language is acquired and the ultimate character of its users' knowledge. Six studies are proposed, four investigating the consequences of early vs. late exposure and two investigating the consequences of impoverished input environments on language acquisition. The studies of early vs. late exposure are intended to follow up on previous findings that age of exposure strongly influences performance, many years later, in both a first and second language. These new studies will investigate whether the effects of age of exposure differ in degree for first vs. second language acquisition, whether they extend to the application of universal syntactic principles in the language, and how they manifest themselves in the initial few years of learning the language. The studies of impoverished of a language are able to reorganize inconsistent aspects of their input and thereby surpass their own input models, acquiring a language rather different than the one to which they were exposed. New studies will investigate the range of linguistic structures for which this is true, and the mechanisms by which such language change occurs. The results should contribute to our understanding of the importance of early experience for language acquisition, and to the character of learning in childhood vs. adulthood. In addition, they should contribute to decisions regarding language exposure, whether spoken or signed, in deaf education and parent counseling.